Bone grafting is widely used to treat fractures, non-unions and to induce arthrodeses. Autogenous cancellous bone, which is taken from one site in the graftee and implanted in another site of the graftee, is considered by many to be the most effective bone graft. Autogeneous cancellous bone provides the scaffolding to support distribution of the bone healing response. Autogeneous cancellous bone also provides the connective tissue progenitor cells which form new cartilage or bone. However, the harvest of autogenous bone results in significant cost and morbidity, including scars, blood loss, pain, prolonged operative and rehabilitation time and risk of infection. Furthermore, in some clinical settings, the volume of the graft required by the graft site can exceed the volume of the available autograft. Accordingly, alternatives to autografts have been developed in an attempt to reduce the morbidity and cost of bone grafting procedures.
The use of allograft bone or xenograft bone is well known in both human and veterinary medicine. See Stevenson et al., Clinical Orthopedics and Related Research, 323, pp. 66-75 (1996). In particular, transplanted bone is known to provide support, promote healing, fill bony cavities, separate bony elements such as vertebral bodies, promote fusion and stabilize the sites of fractures. More recently, processed bone has been developed into shapes for use in new surgical applications, or as new materials for implants that were historically made of non-biologically derived materials.
Because the use of preserved bone intended for implantation to replace diseased or missing parts is common, the successful application of such bone is predicated on sound knowledge of its biologic properties and its capacity to withstand the stresses to which it will be subjected. When mineralized bone is used in grafts, it is primarily because of its inherent strength, i.e., its load bearing ability at the recipient site. The biomechanical properties of bone grafts upon implantation are determined by many factors, including the specific site from which the bone is taken; the age, sex, and physical characteristics of the donor; and the method chosen to prepare, preserve, and store the bone prior to implantation. A more detailed explanation of the alteration of the biomechanical properties of bone by the methods chosen for its preservation and storage may be found in Pelker et al., Clin. Orthop. Rel. Res., 174:54-57 (1983). However, the needs for processing (e.g., to preserve the graft for later use and to remove immunogenic cellular materials) can conflict with the need to conserve the toughness of the bone.
During the preparation of bone intended for implantation the porous matrix is typically contacted with one or more treatment fluids to variously clean, defat, sterilize, virally inactivate, disinfect, and/or demineralize the bone or to impregnate the bone with one or more pharmacological agents (antibiotics, bone growth factors, etc.) so the bone can act as a drug delivery system. See U.S. Pat. No. 5,846,484 for a detailed explanation of the treatment of bone intended for implantation. Some treatment processes, such as irradiation and lyophilization, can work against conservation of the mechanical properties of bone and can lessen the bone's weight bearing properties. Processing requirements can also create dimensional changes in the allograft bone. Such changes of dimension can create damage within the tissue, and may also make it difficult for a machined piece to mechanically engage with surgical instruments, other allografts, or the prepared surgical site. Treatment processes also can have a deleterious effect on such important mechanical properties as toughness. Implants demonstrating improved toughness are important as the insertion of some allografts can be quite energetic, e.g., the hammering in of cortical rings used in spinal fusion surgery.
Bone intended for implantation is currently distributed either frozen or lyophilized. It is generally accepted that freezing monolithic bone to temperatures as cold as −70° C. prior to its packaging and storage results in little if any alteration in its physical properties. However, freezing bone as a preservation technique is costly and can be logistically difficult, e.g., shipping and storage. Lyophilization (freeze-drying, i.e., freezing, then sublimation of moisture) is commonly performed on bone to permit its shelf storage for up to several years without spoilage.
Lyophilization removes excess moisture from the bone and reduces its antigenicity. According to the American Association of Tissue Banks (“A.A.T.B.”), lyophilized whole bone containing no more than 6% moisture can be stored at ambient temperatures for up to five years after processing. However, adverse changes in the biomechanical properties of the bone have been found to result from the lyophilization procedure. Lyophilization can result in damage to the bone due to dimensional changes that occur during the freezing and dehydrating operations. The adverse mechanical changes appear to be associated with damage occurring in the bone matrix, specifically, ultrastructural cracks along the collagen fibers. These effects appear to be magnified when lyophilization and gamma irradiation are used together. Studies using rat bones to model the effects of lyophilization upon the compressive properties of cancellous bone (compression strength of tail vertebrae) and the bending and torsional properties of the long bones indicate that compressive strength can be reduced by up to 30% with little or no change in stiffness, bending strength can be reduced by as much as 40%, and torsional strength can be reduced by up to 60%. These changes, resulting in bone that is brittle, have been found to occur even after the bone has been rehydrated. A more detailed explanation of the effects of lyophilization on mineralized bone can be found in Kang et al., Yonsei Med J 36:332-335 (1995), and Pelker et al., J. Orthop. Res. 1:405-411 (1984).
Thus, a problem exists in providing a bone intended for implantation that is both tough and convenient to store and maintain. Because freezing and thawing bone is minimally damaging to the bone, whereas lyophilization results in reduction in the toughness of the bone, it is the inventors' belief that toughness is maintained and/or enhanced in the bone by dehydrating the bone using methods that remove the water associated with the bone while diminishing the dimensional changes associated with lyophilization. The dehydration of tissue, for example, by treatment with an anhydrous polar organic solvent such as ethanol, is known. See, for example, U.S. Pat. No. 5,862,806 to Cheung. However, it has not been previously appreciated that applying this technique to bone results in an implant having improved biomechanical properties, e.g., toughness. Thus, it is desirable to provide a method for dehydrating monolithic bone intended for implantation prior to its packaging and storage that will better conserve the biomechanical properties of the bone, i.e., its toughness and/or dimensions as compared to lyophilized bone, from the time the bone is harvested through the packaging and storage operations and to time of implantation.